1. Field of the Invention
The present invention relates generally to mobile radio telephone systems. More particularly, the present invention relates to systems and methods for quickly controlling the output transmit power of signals sent from mobile stations to base stations on multiple access channels within a mobile radio telephone system and determining which from among many mobile stations has properly gained access to the multiple access channel.
2. Description of the Related Art
In CDMA radio telephone systems operating in accordance with the TIA/EIA-95 standard (the IS-95 standard), the Access Channel (R-ACH) is used for communications from the mobile station to the base station when the mobile station is not assigned to a dedicated channel such as a Traffic Channel (TCH). The R-ACH carries originations, page responses, registrations, and acknowledgments to messages sent by the base station on the Paging Channel. The R-ACH is transmitted at a constant rate of 4800 bps. This is in contrast to the Traffic Channel which is variable rate. Details of an exemplary CDMA system can be found in U.S. Pat. No. 4,901,307 entitled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeatersxe2x80x9d, which is assigned to the assignee of the present invention and incorporated herein in its entirety by reference. The IS-95 standard is set forth in the TIA/EIA Interim Standard entitled xe2x80x9cMobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, TIA/EIA/IS-95, dated July, 1993, the contents of which are also incorporated herein by reference.
On the R-ACH, the mobile station uses a long code spreading mask that is specific to the base station. In particular, a base station can have up to seven Paging Channels. Associated with each Paging Channel is one or more R-ACHs (up to 32 are permitted). Each R-ACH has a long code mask which determines the spreading sequence. The mask includes the identity of the base station associated with the channel, the Paging Channel to which the R-ACH is associated, and the number of the R-ACH. This provides a unique long code mask and thus a unique long code sequence for the particular R-ACH.
While certainly possible, the R-ACH is not operated in soft handoff. This is unlike the Traffic Channel which is operated in soft handoff. Furthermore, the R-ACH does not have fast power control as on the IS-95 Traffic Channel. On the Traffic Channel, the base station transmits a power control stream at 800 bps to the mobile station. BPSK modulation is used for the power control bit stream. One phase of the bit indicates that the mobile station is to increase its transmit power; another phase of the bit indicates that the mobile station is to decrease its transmit power. The base station determines whether to have the mobile station increase or decrease its transmit power by controlling the received energy to noise density in the base station to a threshold. If the received energy to noise density is less than the threshold, the base station has the mobile station increase its transmit power; if the received energy to noise density is greater than the threshold, the base station has the mobile station decrease its transmit power. This is described more in IS-95 and in U.S. Pat. Nos. 5,056,109 and 5,265,119, both of which are entitled xe2x80x9cMethod and Apparatus for Controlling Transmission Power In A CDMA Cellular Telephone Systemxe2x80x9d and assigned to the assignee of the present invention and incorporated by reference herein.
One reason for not having fast power control on the R-ACH is that multiple mobile stations may transmit on the same R-ACH, thus making it hard to control with one power control stream. Furthermore, if there are many power control streams controlling one channel, it is not clear how to map the power control stream to the mobile station. U.S. Pat. No. 5,604,730 (hereinafter ""730 patent) which is assigned to the applicant describes how one power control stream can be used to control a number of mobile stations. As described below, the technique taught in ""730 patent is also applicable to the invention described herein.
In CDMA systems operating in accordance with the IS-95 standard, the mobile station determines a level at which to transmit on the R-ACH based upon an open loop power control estimate, adjusted by some overhead parameters. More particularly, under the IS-95 standard, the mobile station attempts an access on a R-ACH by sending one or more access probes. An access probe is the message which the mobile station is trying to send to the base station. The mobile station starts by sending an access probe; if the mobile station does not receive an acknowledgment to this access probe, the mobile station increases its transmit power (by a value given in overhead messages) and sends the probe again. This continues until the mobile station receives an acknowledgment or the mobile station has reached the limit of access probes which are permitted.
In any multi-access system, one key aspect of system design is congestion control. From the perspective of the R-ACH, congestion control is responsible for controlling the number of mobile stations simultaneously accessing a R-ACH. Congestion control is important since when too many mobile stations access the system, the system is unable to handle them. Specifically, there can be more transmissions on the reverse link than the base station is able to receive. This is a physical hardware limit. Secondly, the reverse CDMA channel has a capacity limit. When the capacity limit is reached, then the required transmit power of the mobile stations goes to infinityxe2x80x94thus, not permitting communications. Thus, it is necessary to keep the load on the channel within limits. Since the R-ACHs typically share the reverse channel with traffic channels, a certain fraction of the reverse capacity is typically allocated to the R-ACHs. It should be noted that excessive loading on the R-ACHs can create a substantial load on the reverse link, thus limiting the performance of mobile stations which are already assigned to a Traffic Channel. It should also be noted that the R-ACH itself is somewhat unstable as the actual throughput of the R-ACH may decrease after a certain load is reached on the reverse link. In order to control this load, the IS-95 standard has a number of congestion control mechanisms. These include access probe backoffs, access sequence backoffs, channel randomizations, and PN randomizations. However, IS-95 lacks any mechanism for quickly enabling and disabling access to a R-ACH in order to control congestion.
These problems and deficiencies are recognized and solved by the present invention in the manner described below.
One aspect of the invention is directed to a method and apparatus for controlling the transmission of signals from one or more of a plurality of mobile stations to a base station on a plurality of M multiple access channels. A power control information packet formed from a plurality of power control bits is transmitted from the base station to the one or more mobile stations. Each of the power control bits in the power control information packet has a position that is mapped to a selected access channel and to a time offset within the selected access channel. The power control information packet is received at a first mobile station. A message is then transmitted from the first mobile station to the base station on a first access channel and at a first time offset associated with the first access channel. The message is transmitted from the first mobile station at a power level determined in response to a first power control bit in the power control information packet. The first power control bit is located in a first position in the power control information packet, the first position being mapped to the first access channel and the first time offset.
In accordance with a further aspect, the invention is directed to a method and apparatus for controlling the transmission of signals from two or more of a plurality of mobile stations to a base station on a plurality of M multiple access channels. A power control information packet formed from a plurality of power control bits is transmitted from the base station to one or more mobile stations. Each of the power control bits in the power control information packet has a position that is mapped to a selected access channel. The power control information packet is received at a first mobile station and a second mobile station. Messages are then simultaneously transmitted from the first mobile station and the second mobile station to the base station on a first access channel. The messages from the first and second mobile stations are transmitted on the first access channel at power levels that are determined in response to only a first power control bit in the power control information packet, wherein the first power control bit is located in a first position in the power control information packet, and the first position is mapped to the first access channel.
In accordance with a further aspect, the invention is directed to a method and apparatus for controlling the transmission of signals from one or more of a plurality of mobile stations to a base station on one or more of a plurality of multiple access channels. A power control information packet formed from a plurality of power control bits is transmitted from the base station to one or more mobile stations. The power control bits in the power control information packet are transmitted using a modulation that permits each of the power control bits to assume one of first, second and third different states. The power control information packet is received at a first mobile station, and the first mobile station then identifies a state of a first power control bit associated with a first access channel. One of first, second or third operations is then performed at the first mobile station if the state of the first power control bit corresponds to the first state, wherein the first operation corresponds to initiating the transmission of message information from the first mobile station to the base station on the first access channel, the second operation corresponds to inhibiting the initiation of transmission of message information from the first mobile station to the base station on the first access channel, and the third operation corresponds to ceasing the transmission of message information from the first mobile station to the base station on the first access channel. The output power level of the first mobile station on the first access channel is increased if the state of the first power control bit corresponds to the second state, and the output power level of the first mobile station on the first access channel is decreased if the state of the first power control bit corresponds to the third state.
In accordance with yet a further aspect, the invention is directed to a method and apparatus for controlling the transmission of signals from one or more of a plurality of mobile stations to a base station on a plurality of M multiple access channels. A power control information packet formed from a plurality of power control bits is transmitted from the base station to one or more mobile stations. Each of the power control bits in the power control information packet has a position that is mapped to a selected access channel and to a time offset within the selected access channel. The base station next determines whether a mobile station accessed a first access channel during a time interval corresponding to a first time offset. If a mobile station accessed the first access channel during the time interval corresponding to the first time offset, then a first power control bit having a first position within the power control information packet is mapped to the first access channel and the first time offset. If a mobile station failed to access the first access channel during the time interval corresponding to the first time offset, then the first power control bit having the first position within the power control information packet is mapped to the first access channel and a second time offset associated with the first access channel.
In accordance with yet a still further aspect, the invention is directed to a method and apparatus for performing an access channel handoff for a mobile radio unit as the mobile radio unit moves from a first cell in a mobile radio telephone system to a second cell within the mobile radio system. At least one power control bit from a first base station associated with the first cell is transmitted to the mobile radio unit. The power control bit from the first base station is transmitted at a first time interval that corresponds to a first access channel associated with the first base station. At least one power control bit from a second base station associated with the second cell is also transmitted to the mobile radio unit. The power control bit from the second base station is transmitted at a second time interval, that may correspond to the first time interval used by the first base station. The power control bit from the second base station that corresponds to the access channel that was associated with the first base station. Each of the power control bits in the power control information packet has a position that is mapped to a selected access channel and to a base station associated with the selected access channel. The mobile radio unit receives the power control packet from the first base station and the power control packet from the second base station, determines the corresponding power control bits, and then determines the state of the power control bits. If the state indicates that a message is to be transmitted and the mobile station has a message to transmit, then the mobile station transmits the message on a selected access channel. Each of a set of prescribed base stations attempts to receive the transmission on the selected access channel and then sets the corresponding bit in the power control information packet in accordance with the received signal to noise ratio.